The last panels are fairly lazy, as well XD .

Rather than drawing the backgrounds every time, it's sometimes more effective to use speed lines or other atmospheric effects to set the right mood.

When it comes to a conversation, it‘s important to concentrate on the characters facial and bodily expressions. The speech bubbles usually take up a lot of space, too, so detailed backgrounds would be dead weight in this case.

In these instances it's often best to either leave the bg‘s blank or use screen tones (in some cases even a solid black fill will do the job).